When a Servant is summoned by the Holy Grail, they are gifted with the knowledge of the modern world so they can easily adjust. When it comes to determining the strengths and weaknesses of an enemy Servant, the Servants themselves seem to be able to rely on a pool of data, as shown during the prologue of Fate/Stay Night: when Rin and Archer face Lancer, Archer reveals that he has deduced Lancer's true identity.
However, during this same scene, Lancer is frustrated because he doesn't know of any hero who was an Archer but also used twin swords, given that we know that Archer is
a future alternate version of Shirou who became the Counter-Guardian EMIYA.
When a Servant is summoned, are they also given the knowledge of the legends for heroes who appeared after that Servant's death? (i.e. Gilgamesh is probably the oldest legend, and as such he predates most others and thus wouldn't know about other legends that occur after his death unless supplemented by the Grail)